The platinum group metals (PGM) or PGMs (platinum, palladium, rhodium, iridium, osmium, and ruthenium) are becoming increasingly important to the global economy. Platinum is used for jewelry and chemical catalysts, and blends of platinum, palladium, and rhodium are used in catalytic converters.
PGMs frequently occur together in naturally occurring sulfide and silicate minerals along with iron and base metals, such as, nickel, copper, and cobalt. By way of illustration, a typical PGM sulfide concentrate contains from about 0.1 to about 15 oz/ton platinum, from about 0.1 to about 15 oz/ton palladium, from about 0.01 to about 3 oz/ton rhodium, and also contains variable amounts of nickel, copper, magnesium, cobalt, chromium, and iron. Typically, a PGM sulfide concentrate includes from about 2 to about 30 wt % iron, from about 1 to about 25 wt % magnesium, from about 0.1 to about 15 wt % nickel, from about 0.05 to about 10 wt % copper, from about 0.001 to about 0.5 wt % cobalt, from about 0.1 to about 10 wt % chromium, and from about 0.1 to about 20 wt % sulfur. The nickel and copper are typically present as sulfides, the magnesium as silicates, the iron as both sulfides and silicates, and the chromium as chromium (III) oxide as a chromite, Cr2O3, also known as a spinel, which can form spinel-like compounds of the form MeOX.Cr2O3, where X is a real number usually less than 1.0 and Me is a divalent cation typically Mg or Ca. Minerals typically containing one or more of alkali or alkali earth elements such as sodium or magnesium, respectively, as well as chromium, aluminum, and iron, all in oxide form, usually in association with SiO2 are hereinafter referred to as “gangue elements” or “gangue constituents” Normally, the ratio of gangue-to-sulfide is from about 2:1 to 60:1.
FIG. 1 depicts a typical process for recovering platinum group metals and base metals. The PGM-containing material 110 is processed, the processed PGM-containing material is melted 120, to form a furnace matte (containing the PGMs and base metals as sulfides), the furnace matte is converted 130 to form a converter matte (containing the PGMs and base metals), and the converter matte is processed by hydrometallurgical methods 140 to isolate, purify and recover individual base 150 and platinum group 160 metals.
Step 120 will be discussed in more detail with reference to FIG. 4. The unmelted material in zone 420 becomes partially liquid at the interface 414 of the unmelted material 420 and the slag 408, forming three phases: matte, slag and trivalent oxides. The partially liquefied material moves into slag 408 where the matte, slag, and trivalent oxide skeleton phases separate. The dense sulfide-containing matte phase moves into the matte 412. The matte 412 contains most of the PGMs, some of the iron, and some of the chromium. The lighter slag phase and trivalent oxides remain in the slag 408. The slag is silicate-rich and contains gangue elements, including chromium (Cr(II) and Cr(III)) oxides (such as CrO and/or Cr2O3) and iron (Fe(II) and Fe(III)) oxides (such as FeO, Fe3O4 or Fe2O3), and some PGMs and base metals. The trivalent chromium, Cr (III) and trivalent iron Fe (III), if present, form a separate spinel-like solid phase structure in the slag.
To the extent that the trivalent oxides form faster than the rate of removal through the slag tap holes they accumulate in slag 408 and eventually invade the matte 412 and some settle in zone 416, typically referred to within the art as “mush”, “magnetite”, “bottom” or “false bottom”, owing to its tar-like consistency. The accumulation of the trivalent oxides reduces the effective working volume of the furnace crucible and reduces the time available for phase separation and settling of the matte.
A clean separation of the slag from the matte is desired to provide optimal PGM recovery; however, such a clean separation is difficult to achieve in the presence of trivalent metals, such as trivalent chromium and trivalent iron. Because the trivalent metals, especially trivalent chromium have a limited solubility in the slag, it is believed that they form a skeleton-like solid phase and cause the slag to be more viscous, which inhibits separation of the matte and slag phases and tapping of the slag from the furnace. It is also believed that the trivalent oxide skeletons trap droplets of matte, which, together with the poor phase separation, are the major causes of PGM and other value metal losses in the slag.
To improve slag fluidity and phase separation, smelters have used slag additives (e.g., fluxing agents), higher operating temperatures, and increased electromagnetic agitation. Another measure includes limiting the amount of chrome contained in the concentrate. While these measures have been helpful, current practice limits the chrome content of concentrates and exploitation of high chrome content deposits. In those deposits and concentrates having acceptable chromium oxide levels, the use of higher operating temperatures and electromagnetic agitation impact adversely the efficiency and economics of the PGM recovery process. Electromagnetic agitation of the slag leads to undesirable temperature equalization between the matte and slag, which effectively increases the matte superheat. The superheat is the difference between the actual temperature and the melting point of the matte or slag. This lowers the viscosity and increases the mobility of the matte which in turn increases mass and thermal fluxes to the walls and increases wear of the containment system. The current smelting process for high chromium PGM ores is further believed to shorten furnace life and result in high maintenance costs and more frequent and lengthy repair periods.